The present invention pertains to a method for transporting preforms in the area of a blow-molding machine for the blow-molding of containers of thermoplastic material along a sorting device configured as a roll conveyor with two rotating, in particular counter-rotating, conveying rolls; to a device for controlling a method for transporting preforms in the area of a blow-molding machine; and to a device for the blow-molding of containers of thermoplastic material.
So that preforms can be fed to a blow-molding machine, the preforms are typically transported along a slide rail with their openings facing upward. The preforms are guided and supported typically by support rings, which are arranged underneath the threads of the preforms. These support rings are also called neck rings. The slide rail is usually arranged with a steep downward slant toward the blow-molding machine, so that a back pressure develops in the row of transported preforms.
To ensure that blow-molding machines with high production capacity can be supplied reliably with preforms, relatively long slide rails are used to achieve an adequate back pressure of the preforms in the entrance area to the blow-molding machine in spite of the frictional forces which occur during the movement of the preforms along the slide rails.
To singulate and sort the preforms, a roll conveyor can be installed upstream of the slide rail. An ascending conveyor is typically installed upstream of the roll conveyor. The ascending conveyor accepts preforms from a supply container or silo, conveys them upward, and transfers them, unsorted and unoriented, to the roll conveyor. The ascending conveyor usually operates on the principle of a conveyor belt.
The roll conveyor singulates and sorts the unorganized preforms. The roll conveyor orients the preforms, so that they can be suspended by their support rings between two approximately parallel rolls and can be moved forward in a row, with their openings facing upward, in the longitudinal direction of the conveying rolls. The roll conveyor is arranged at an angle and extends from the ascending conveyor down toward the slide rail, so that the preforms slide along the slanted rolls by the force of gravity toward the slide rail. By the time they reach the end of the roll conveyor, the preforms are oriented with their openings facing upward and are arranged in a row, one behind the other, so that they can be transferred to the slide rail in orderly fashion.
During the transport of the preforms within the roll conveyor, gaps occur, which must be filled by the time the preforms reach the blow-molding machine, so that there will not be any interruption in the feed to the machine. For this purpose, it is known that, for example, a queue brake arranged in the area of the roll conveyor can produce additional back pressure in the row of preforms, so that the incoming preforms can slide forward and fill the gap.
So that the gaps can be filled at an early point by active control of the transport device or roll conveyor, the gaps must first be detected reliably. It is known from the prior art that gaps can be detected by means of a time-resolved measurement. For this purpose, light barrier sensors are typically placed at a few separate locations. The beam paths of the light barriers typically pass through the intermediate space between the conveying rollers more-or-less crosswise to the longitudinal direction of the rollers. As soon as a preform being transported between the conveying rollers interrupts the light barrier, the light barrier sensor generates a signal. Gaps between the transported preforms are recognized when no preform is detected at the individual position in question for a certain period of time.
The disadvantage of the prior art described above is that the light barriers monitor only certain individual positions, and any gaps which may be present outside these monitoring positions are not recognized at all or only after a considerable delay. As a result, supply fluctuations occur in the transport process, which can lead to control deviations. This can then lead in turn to the danger of interruptions in the sorting stream and to interruptions in the operation of the blow-molding machine.